5. Implications of detection at high redshift
Any present at high redshifts has to have been synthesized in massive stars only. Timmes et al. (1997) have argued further that its detection at redshifts would in fact be a signature of the -process in massive star explosions. The possibility of production by non-exploding WR stars might in fact weaken this statement, and blur the picture substantially.
Of course, one has to acknowledge that the contribution from WR stars at high redshifts may be reduced as a direct result of the lower metallicities that appear to characterize such regions. According to observations of Damped Lyman systems (Pettini et al. 1997), the metallicity at redshifts between 1.5 and 2 indeed lies around . Such a reduced metallicity lowers the WR yields for two reasons. First, the number of WR stars predicted by non-rotating single star models is considerably reduced as a result of lower mass losses (Maeder & Meynet 1994). Second, the abundances of the CNO seeds that are needed for the secondary WR production are reduced as well.
Even so, it would certainly be premature at this point to completely forget about the role of WR stars in a possible enrichment of high-z material with , and to relate it strictly with the -process.This is even more true as the predictions reported in this paper are based on single, non-rotating stellar models only. How binarity and/or rotation would change these results remains to be checked. At present, the published rotating evolutionary models leading to WR stars (Fliegner & Langer 1994, Meynet 1998, 1999) make no predictions concerning the synthesis of fluorine. However they show that rotation favours an early entrance into the WR phase for a given mass, and decreases the minimum initial mass for a star to go through a WR phase at a given metallicity. Moreover, the mixing induced by rotation opens up new nucleosynthetic channels (see Heger 1998) whose importance for the scenario of fluorine production presented in this paper remains to be quantitatively assessed. Finally, let us note that the effects of rotation might be more important at low Z if, as suggested by Maeder et al. (1999), the average rotation is faster at low metallicities. In such conditions, and in absence of quantitative calculations, one has to remain alert to the possibility of a significant contamination of low metallicity high redshift regions by the -loaded wind of WR stars.
Clearly, observations of at high redshift, if possible at all, would be decisive in order to answer the question of the very production mechanism of this element. An important distinguishing feature would be the primary nature of the observed , as predicted by the -process, or its secondary behaviour, as expected from the thermonuclear model discussed in this paper.
© European Southern Observatory (ESO) 2000
Online publication: March 17, 2000